Lithographic projection exposure methods are predominantly used nowadays for producing semiconductor components and other finely structured components, such as, for example, masks for photolithography. In this case, use is made of masks (reticles) or other patterning devices that bear or form the pattern of a structure to be imaged, for example a line pattern of a layer of a semiconductor component. The pattern is positioned in a projection exposure apparatus between an illumination system and a projection lens in the region of the object plane of the projection lens and is illuminated with an illumination radiation shaped by the illumination system. The radiation modified by the pattern travels through the projection lens as projection radiation, said projection lens imaging the pattern with a reduced scale onto the substrate to be exposed. The surface of the substrate is arranged in the image plane of the projection lens optically conjugate to the object plane. The substrate is generally coated with a radiation-sensitive layer (resist, photoresist).
One of the aims in the development of projection exposure apparatuses is to lithographically produce structures having smaller and smaller dimensions on the substrate, for example to obtain greater integration densities in semiconductor components. One approach consists in working with shorter wavelengths of the electromagnetic radiation. By way of example, optical systems have been developed which use electromagnetic radiation from the extreme ultraviolet range (EUV), in particular having operating wavelengths in the range of between 5 nanometers (nm) and 30 nm, in particular of 13.5 nm.
An EUV projection exposure apparatus including an illumination system is known e.g. from U.S. Pat. No. 7,473,907 B2. The illumination system is designed for receiving EUV radiation of an EUV radiation source and for shaping illumination radiation from at least one portion of the received EUV radiation. The illumination radiation is directed into an illumination field in an exit plane of the illumination system during exposure operation, wherein the exit plane of the illumination system and the object plane of the projection lens advantageously coincide. The illumination radiation is characterized by specific illumination parameters and is incident on the pattern within the illumination field with a defined position, shape and size at defined angles. The EUV radiation source, which may be a plasma source, for example, is arranged in a source module separate from the illumination system, said source module generating a secondary radiation source at a source position in an entrance plane of the illumination system.
Arranged in a housing of an illumination system of the type considered here are a plurality of mirror modules, which are each located in the final installed state at installation positions that are provided for the mirror modules. The mirror modules or reflective mirror surfaces of the mirror modules define an illumination beam path extending from the source position to the illumination field. The mirror modules include a first mirror module having a first facet mirror at a first installation position and a second mirror module having a second facet mirror at a second installation position of the illumination system. A mirror module of this type has a main body acting as a carrier, on which facet elements with reflective facets are mounted alone or in groups in accordance with a specific local distribution.
If the reflective facets of the first facet mirror are situated at or near a field plane of the illumination system that is conjugate to the exit plane, the first facet mirror is frequently also referred to as a “field facet mirror.” Correspondingly, the second facet mirror is frequently also referred to as a “pupil facet mirror” if the reflective facets thereof are situated at or near a pupil plane which is Fourier-transformed with respect to the exit plane.
The two facet mirrors contribute in the illumination system of the EUV apparatus to the homogenization or mixing of the EUV radiation.
DE 10 2012 209 412 A1 makes note of the fact that, in facet mirrors for EUV applications, the correct angular orientation of the individual facets of each facet mirror is important for the quality of the beam shaping. The emphasis is here on the fact that the individual facets of an individual facet mirror can have different angular orientations relative to one another and that each angular orientation of each facet must be correctly adjusted for the proper function of the facet mirror in the optical system. Therefore it is considered desirable for the angular orientations of the facets to be adjusted with the correct orientation during the production of a facet mirror, which assumes a corresponding precise measurement of the angular orientations of the individual facets. Proposed for this purpose is an optical method for measuring angular orientations of facets of at least one facet mirror of an optical system designed for EUV applications and for subsequently adjusting the angular orientations in dependence on the measured angular orientations. In this method, the facets of the facet mirror are illuminated with measurement light. The measurement light which is reflected by the facets is detected and evaluated to obtain current angular orientations, wherein the angular orientations are subsequently adjusted if the current angular orientations deviate from desired angular orientations. The angular orientations of the facets are obtained in a spectrum of angular orientations of at least ±10° with respect to a reference axis. The method can be performed both on a facet mirror which has been demounted from the illumination system and also on a facet mirror which is mounted at its installation position in the illumination system.
The method permits measurement and setting of angular orientations of facets of the facet mirrors with great precision.
WO 2010/008993 A1 describes illumination systems having facet mirrors, in which the individual facets are tiltable so as to be able to set for example a different illumination setting during the operation of the projection exposure apparatus by tilting individual facets or all facets. Also described therein is a measurement apparatus for permitting measurement of the angular orientations of the tiltable facets during the operation of the projection exposure apparatus. To this end, the measurement apparatus has for each facet mirror a measurement light source and a detector, which is in the form of a Shack-Hartmann sensor.